Our studies have been aimed at determining the molecular mechanism by which reticulum cell sarcoma (RCS) stimulate syngeneic lymphocytes from SJL/J mice. Our preliminary studies have shown that antigens coded for by the I region of the MHC are expressed on RCS tumor cells but not on host cells. These antigens are recognized by the anti-tumor lymphocytes and are also recognized by certain anti-allogeneic lymphocytes. We propose to investigate the role of the tumor antigens in the induction of the anti-tumor response and in the regulation of the host anti-tumor response. Monoclonal antibodies will be used. Furthermore, the role of suppressor cells in regulation will be defined.